Secondary station and method of operating the station

ABSTRACT

A secondary station has a receiver capable of resolving signals received as a plurality of multipath signals from a plurality of base stations during a soft handover process. This capability may, for example, be provided by a Rake receiver. In order to decode and act upon the received signals in a very short period of time, signals ( 402   g,   402   h ) arriving after a predetermined time are not processed by the receiver which is able instead to process weaker signals ( 402   d,   402   e ) which arrived before the time (t 1 ).  
     Such a secondary station is particularly suitable for decoding and acting upon power control commands included in received signals in a UMTS system, for which a very limited period of time is provided by the UMTS specification. In this application the predetermined time (t 1 ) is the time after which signals are received too late for use in determining the next power control change

[0001] The present invention relates to a secondary station for use in aradio communication system and further relates to a method of operatingthe secondary station. While the present specification describes asystem with particular reference to the emerging Universal MobileTelecommunication System (UMTS), it is to be understood that thetechniques described are equally applicable to use in other mobile radiosystems.

[0002] There are two basic types of communication required between aBase Station (BS) and a Mobile Station (MS) in a radio communicationsystem. The first is user traffic, for example speech or packet data.The second is control information, required to set and monitor variousparameters of the transmission channel to enable the BS and MS toexchange the required user traffic.

[0003] In many radio communication systems accurate power control isimportant. This is particularly so in systems employing spread spectrumCode Division Multiple Access (CDMA) techniques, because manycommunication channels share the same bandwidth and so transmission attoo high a power in any one channel reduces the signal to noise ratio inall the other channels. Uplink power control, of signals transmitted toa Base Station (BS) from a Mobile Station (MS), is particularlyimportant. It ensures that the BS receives signals from different MSs atapproximately the same power level for a given data rate and quality ofservice, while minimising the transmission power required by each MS.Downlink power control, of signals transmitted by the BS to a MS, isrequired so that the MS receives signals from the BS with a low errorrate while minimising transmission power, to reduce interference withother cells and radio systems.

[0004] In a UMTS embodiment, power control is normally operated in aclosed loop manner. For uplink power control the BS determines therequired changes in the power of transmissions from a MS and signalsthese changes to the MS by means of Transmit Power Control (TPC)commands. To minimise overheads, a TPC command typically instructs theMS to increase or decrease its power, with the change in power being astep of predetermined size. However, in some systems a TPC command mayalso determine the step size to be used.

[0005] A MS generally communicates with a single BS. During the courseof a call the MS may wish to investigate transferring to another BS, forexample when the quality of the communication link deteriorates as theMS moves away from its BS, or when the relative traffic loading ofdifferent cells requires adjusting. The process of transferring from oneBS to another is known as handover. In a version of this process knownas soft handover, the MS engages in communication with a plurality ofBSs (known as the “active set” of BSs) to determine to which BS, if any,it should transfer. When the MS is engaged in this process it willreceive TPC commands from each of the BSs. An example of a strategy fordetermining what change in power to make based on the received TPCcommands is disclosed in International Patent Application WO 00/36762.

[0006] A problem with power control during soft handover is that thereis a limited amount of time available to receive, decode and process thepower control commands. For example, in UMTS there is a period of 416chips (approximately 108 μs) after the arrival of the first TPC commandduring which the received commands need to be decoded and processed todetermine the magnitude and direction of the required power change. Thisperiod is followed by a period of 50 μs during which the transmissionpower change should be made.

[0007] This problem is made worse because in UMTS soft handover theremay be a time difference of up to 148 chips (38.5 μs) between thearrival times of the first signals from each BS. When the signal from aBS is received via several downlink paths and the information from thepaths is combined (for example using a Rake receiver), a further delayis introduced of up to the worst-case delay spread between paths. In aUMTS system this could reduce the available processing time by up to 20μs. The combined effect of soft handover and delay spread can thereforebe to reduce the available processing time by half. This allows verylittle flexibility for scheduling of the necessary processing tasks in areceiver, particularly for transceiver architectures having significantprocessing delay through use of vector processors.

[0008] An object of the present invention is therefore to maximise thetime available in a MS for processing of power control commands.

[0009] According to a first aspect of the present invention there isprovided a secondary station for use in a radio communication systemcomprising a plurality of primary stations, the secondary station havingmeans for engaging in a soft handover process, in which the secondarystation communicates simultaneously with at least two primary stations,and receiver means including signal resolution means for resolvingtransmitted signals from the at least two primary stations received as aplurality of multipath signals, wherein means are provided fordetermining whether to allocate each multipath signal to the signalresolution means based on its time of arrival.

[0010] According to a second aspect of the present invention there isprovided a method of operating a secondary station comprising engagingin a soft handover process, in which the secondary station communicatessimultaneously with at least two primary stations, receiving transmittedsignals from the at least two primary stations as a plurality ofmultipath signals, and processing the received multipath signals by asignal resolver to resolve the transmitted signals, the method furthercomprising determining whether to process each multipath signal based onits time of arrival.

[0011] The present invention is based upon the recognition, not presentin the prior art, that allocation of signals to Rake receiver fingersbased on their time of arrival, rather than their strength, enablesimproved implementation of a secondary station.

[0012] Embodiments of the present invention will now be described, byway of example, with reference to the accompanying drawings, wherein:

[0013]FIG. 1 is a block schematic diagram of a radio communicationsystem;

[0014]FIG. 2 is a block schematic diagram of a Rake receiver having fivefingers;

[0015]FIG. 3 is a block schematic diagram of a radio communicationsystem with a MS in the process of soft handover;

[0016]FIG. 4 is a graph showing the amplitude A of received signalsagainst time t from receipt of the first signal; and

[0017]FIG. 5 is a flow chart showing a method of processing multipathsignals in accordance with the present invention.

[0018] In the drawings the same reference numerals have been used toindicate corresponding features.

[0019] Referring to FIG. 1, a radio communication system comprises aprimary station (BS) 100 and a plurality of secondary stations (MS) 110.The BS 100 comprises a microcontroller (μC) 102, transceiver means(Tx/Rx) 104 connected to antenna means 106, power control means (PC) 107for altering the transmitted power level, and connection means 108 forconnection to the PSTN or other suitable network. Each MS 110 comprisesa microcontroller (μC) 112, transceiver means (Tx/Rx) 114 connected toantenna means 116, and power control means (PC) 118 for altering thetransmitted power level. Communication from BS 100 to MS 110 takes placeon a downlink frequency channel 122, while communication from MS 110 toBS 100 takes place on an uplink frequency channel 124.

[0020] The transceiver means 114 in a MS 110 may include a Rakereceiver. Such a receiver, well known to those skilled in the art, isdesigned to detect a CDMA signal transmitted over a dispersive multipathchannel. A block schematic diagram of a five-finger Rake receiver isshown in FIG. 2. Signals received via the antenna 116 are down-convertedto baseband and supplied as a first input to five mixers 202. A signalgenerator (SIG) 204 generates a local copy of a signal encoded with thesame spreading code as that used by the BS 100. This signal is suppliedas a second input to the first mixer 202 a. The same signal, delayed bydelay means (DEL) 206 b, is supplied as a second input to the secondmixer 202 b, and similarly to mixers 202 c, 202 d, 202 e delayed furtherby delay means 206 c, 206 d, 206 e respectively.

[0021] By adjusting the phase of the generated signal according to theoutput of a channel estimator and the delays applied by the delay means206, five versions of the same transmitted signal received by fivedifferent paths having different delays can be handled. The receivedsignals have their amplitudes multiplied by a weight factor proportionalto their respective received signal strengths by attenuators (ATT) 208,and are then summed by adding means (SUM) 210. The combined signal isthen integrated by integration means (INT) 212 over successive symbolperiods to determine the received symbols, which symbols are supplied tothe remainder of the receiver for further processing. If signals arereceived via more than five different paths, the phase of the signalgenerator 204 and the delays introduced by the delay means 206 areadjusted to match the five strongest received paths (or those with thebest signal to interference ratio).

[0022] A MS 110 engaged in a soft handover process is illustrated inFIG. 3, the MS 110 having three two-way communication channels 326 a,326 b, 326 c with three respective BSs 100 a, 100 b, 100 c. In a giventime slot the MS 110 receives TPC commands from each of BSs 100 a, 100b, 100 c. If the received signals are processed by a Rake receiverhaving n fingers, it is conventional for each of the n strongest signalsto be allocated to a finger.

[0023] An example of a set of signals received via different paths isshown in FIG. 4 as a graph of the amplitude A of a plurality of signals402 against the time of arrival t relative to the time of arrival of thefirst received signal 402 a. If the signals are processed by a sixfinger Rake receiver, Rake fingers would be allocated to signals 402 a,402 b, 402 c, 402 f, 402 g and 402 h, while signals 402 d and 402 ewould be ignored.

[0024] However, the applicants have determined that such an allocationstrategy may not be optimum during soft handover, in view of the limitedprocessing time available. Instead it is proposed, in accordance withthe present invention, that Rake fingers are allocated to signals basedon time of arrival information, either alone or in combination withsignal strength information. This overcomes the situation that a Rakefinger is allocated to a signal which has arrived too late to beincorporated in the decision-making process for the next power change,in which case such a finger is effectively wasted.

[0025] Hence, in the example shown in FIG. 4, the MS 110 determines thatany signals received after time t₁ will be too late to be used indetermining the next power change. Consequently, the allocation of Rakefingers may be modified from that normally used, as described above,with the fingers being allocated instead to signals 402a, 402 b, 402 c,402 d, 402 e and 402 f. This allocation therefore ignores strong latesignals (402 g, 402 h), which arrive after a predefined window ofarrival for the first significant path. Such late-arriving signals couldoptionally be used in determining the power control change to be made ina subsequent slot.

[0026] The MS 110 may also employ additional techniques, in addition toor instead of the modified allocation of Rake fingers described above,to increase the time available for processing power control commands.One such technique is for the MS 110 to start making its power change onthe basis of an initial estimate of the required power change, madebefore all information from received signals is available. If necessary,the power change could then be corrected based on further receivedsignals. This technique would yield benefits in flexibility forscheduling of processing tasks in the MS 110, provided that a correctionto the implemented power change was only required in a small proportionof cases.

[0027] In some soft handover situations, for example when a reliabledown command is received from the earliest BS 100, there is no need towait for further power control commands to arrive. In other cases, asuitable strategy might be always to reduce power when a down command,whether reliable or not, is received from the earliest BS 100, andsimilarly for increasing power in response to an up command subject tothe power change being corrected in the event that a reliable downcommand is received later. Such a strategy would meet the requirementsof the UMTS specification.

[0028] On occasions when the direction of the power control step didrequire correction, such correction would be likely to extend beyond theallocated 50 μs period for making power changes. In itself this is not aproblem, provided that the average uplink transmission power for theremainder of the slot after the power change is not affected to such anextent that it falls outside permitted tolerances, and provided that theerror vector magnitude does not exceed permitted tolerances. The errorvector magnitude is defined in UMTS as the root mean square (rms) errorvector between the transmitted waveform and a closely-matched referencewaveform.

[0029] In systems other than UMTS, particularly where information fromdifferent BSs 100 during soft handover might affect the requiredmagnitude of the power step, the strategy of starting the power changebased on an initial estimate could have greater benefits. For example,the power change could be implemented by means of an initial coarsepower change in the RF part of the transceiver 114 with the remainingfine tuning of the transmission power being achieved by adjusting theamplitude of the baseband signals for transmission once the remainingpower control commands had been processed.

[0030] Another technique is to modify the timings of transmissions froma BS 100. According to the UMTS specification, the MS 110 notifies a BS100 if the time of arrival of its signals drifts outside a range oftypically ±148 chips relative to signals from other BSs 100 (oralternatively relative to a fixed offset from the timing reference foruplink transmissions). The BS 100 can adjust its transmission timing insteps of 256 chips. By reporting a received signal as having arrivedoutside the acceptable time limit, even if it has not, the MS 110 canarrange the timing of downlink transmissions from a plurality of BSs 100so as to improve power control command processing. The MS 110 may alsodecide not to report a weak signal which arrives outside the time limit,for example to avoid any consequent changes to the timing reference.

[0031] For example, if the signal from the first-received BS 100 wasconsistently weaker than the signals from other BSs 100 received later,the MS 110 could report one or more of the stronger, later signals ashaving arrived late, so that the UMTS network arranged for its timing tobe advanced by 256 chips, thereby ensuring that the strongest signal wasreceived first. This algorithm would significantly improve applicationof the initial estimation method described above.

[0032] As an alternative, or in addition, to the above technique, the MS110 could report as late any downlink signal received more apredetermined amount late, for example 74 chips (i.e. half of the 148chip timing tolerance), so as to maximise the time available for powercontrol command processing.

[0033] A flow chart illustrating a method in accordance with the presentinvention of processing multipath signals is shown in FIG. 5. The methodstarts, at step 502, with a MS 110 beginning a soft handover process.The MS 110 continues, at step 504, to receive multipath signals from aplurality of BSs 100 until the time t passes the latest time for signalsto be taken into account in determining the next transmission powerchange. When this time is reached, test 506 is passed and the receivedsignals are allocated to fingers of a Rake receiver at step 508. Ifthere are more received signals than available Rake fingers, theallocation is made on the basis of signal strength or other suitablefactors. The signals are then processed, at step 510, by the Rakereceiver, after which the required power change can be determined andimplemented, at step 512.

[0034] Although the above description relates to a Rake receiver, itwill be apparent that the present invention is equally applicable to anyreceiver capable of resolving a plurality of multipath signals. Further,although the above description relates to reception of power controlcommands via a plurality of multipath signals the present invention isalso applicable to other transmissions having tight time constraints fordecoding. An example of such a transmission is feedback information forcontrolling transmit diversity of BSs 100 in a UMTS system.

[0035] From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the design, manufacture anduse of secondary stations and component parts thereof, and which may beused instead of or in addition to features already described herein. Itwill be appreciated that certain features of the invention which are,for clarity, described in the context of separate embodiments may alsobe provided in combination in a single embodiment. Conversely, variousfeatures of the invention which are, for brevity, described in thecontext of a single embodiment may also be provided separately or in anysuitable subcombination. Although claims have been formulated in thisapplication to particular combinations of features, it should beunderstood that the scope of the disclosure of the present applicationalso includes any novel feature or any novel combination of featuresdisclosed herein either explicitly or implicitly or any generalisationthereof, whether or not it relates to the same invention as presentlyclaimed in any claim and whether or not it mitigates any or all of thesame technical problems as does the present invention. The applicantshereby give notice that new claims may be formulated to such featuresand/or combinations of features during the prosecution of the presentapplication or of any further application derived therefrom.

[0036] In the present specification and claims the word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements. Further, the word “comprising” does not exclude thepresence of other elements or steps than those listed.

1. A secondary station for use in a radio communication systemcomprising a plurality of primary stations, the secondary station havingmeans for engaging in a soft handover process, in which the secondarystation communicates simultaneously with at least two primary stations,and receiver means including signal resolution means for resolvingtransmitted signals from the at least two primary stations received as aplurality of multipath signals, wherein means are provided fordetermining whether to allocate each multipath signal to the signalresolution means based on its time of arrival.
 2. A secondary station asclaimed in claim 1, characterised in that the signal resolution meanscomprises a Rake receiver having a plurality of fingers.
 3. A secondarystation as claimed in claim 1, characterised in that power control meansare provided for adjusting the power of uplink transmissions in responseto power control commands received from the primary stations and in thatthe signal resolution means are operable to resolve power controlcommands received via a plurality of multipath signals.
 4. A secondarystation as claimed in claim 3, characterised in that the power controlmeans adjusts transmission power at predetermined times and in that latesignals, having a time of arrival which is less than a predeterminedinterval before the next time for adjustment of transmission power, arenot allocated to the signal resolution means.
 5. A secondary station asclaimed in claim 4, characterised in that at least some of the latesignals are used in the determination of a subsequent power controladjustment.
 6. A method of operating a secondary station comprisingengaging in a soft handover process, in which the secondary stationcommunicates simultaneously with at least two primary stations,receiving transmitted signals from the at least two primary stations asa plurality of multipath signals, and processing the received multipathsignals by a signal resolver to resolve the transmitted signals, themethod further comprising determining whether to process each multipathsignal based on its time of arrival.
 7. A method as claimed in claim 6,characterised by the signal resolver being a Rake receiver having aplurality of fingers.
 8. A method as claimed in claim 6, characterisedby adjusting the power of uplink transmissions in response to powercontrol commands received from the primary stations and by resolvingpower control commands received via a plurality of multipath signals. 9.A method as claimed in claim 8, characterised by adjusting transmissionpower at predetermined times and by late signals, having a time ofarrival which is less than a predetermined interval before the next timefor adjustment of transmission power, not being processed by the signalresolver.
 10. A method as claimed in claim 9, characterised by at leastsome of the late signals being used in the determination of a subsequentpower control adjustment.